Mooring systems of the type to which the present invention is related consist essentially of a plurality of lengths of chain having anchors on the outboard ends thereof. The drilling rig or other instrumentality which the anchors and chains moor include chain handling mechanisms which enable the anchors to be deployed to render the system operational. Typically, each chain handling mechanism is used in conjunction with a chain locker within which the inboard end portion of the length of chain is stored. From the chain locker each chain extends in cooperative relation with an anchor windlass assembly. From the anchor windlass assembly, the chain extends in cooperating relation with a fairlead assembly. A typical semi-submersible drilling rig may involve the provision of three vertically extending tubular chain lockers which also provide flotation for the rig. Each chain locker has three anchor windlass assemblies associated therewith which normally are disposed at the platform level of the rig. Three cooperating fairlead assemblies are mounted in a submersible position below the anchor windlass assemblies. The mooring system thus provides nine lengths of chain and nine anchors which are spaced peripherally outwardly from the moored position of the rig.
It is sometimes necessary where emergency conditions arise, as for example when severe storms and hurricanes are impending, to release the rig from its mooring system. When these emergency conditions arise, it is simply not practical to attempt to haul in each of the nine anchors and lengths of chain. Various somewhat complex arrangements have been proposed for quickly severing the lengths of chain extending from the fairlead assemblies to the anchors. Examples of prior art devices of this type are disclosed in U.S. Pat. Nos. 3,905,190, 4,033,277 and 4,067,282. In all of these arrangements the approach is to include a specially constructed releasable connecting device in the length of chain that is disposed outboard of the associated fairlead assembly. The releasable connecting device normally functions effectively as a connector between two links of the chain, but is operable in response to a predetermined signal to separate the two links in the chain which it serves to connect. Because the releasable connecting device must remain in the water, it is constantly subjected to the harsh environment of sea water and as a result often fails to operate. Furthermore, the device in U.S. Pat. No. 4,067,282 for receiving the predetermined signal is deployed externally from the link, and as a result stoppers are required to ensure that the associated link is not passed through the fairlead and windlass assemblies, so that the external device is not damaged or more importantly to prevent the explosive device from being detonated while passing through the aforementioned assemblies. Accordingly, the handling and deployment of the U.S. Pat. No. 4,067,282 link is difficult, complex and involved in that it is designed not to pass through the chain handling assemblies.
In commonly assigned copending application Ser. No. 582,469 filed Feb. 22, 1984 there is disclosed a system which obviates the disadvantages noted above without providing offsetting disadvantages. As disclosed, the system includes at least one separable link in the length of chain which is of different construction than the remaining links, but yet is of a construction similar to the remaining links such that it will move in cooperating relation through the chain handling mechanism including the anchor windlass assembly and the fairlead assembly. Each such one link includes a plurality of removably interconnected parts operable when interconnected to interengage with adjacent links of the associated length of chain. The parts of each one link are interconnected either by manually removable fasteners or by one or more frangible fasteners. The interconnected parts of each link are normally stored in the chain locker interengaged with adjacent links. When it becomes necessary to effect an emergency release of the mooring system, an explosive bolt assembly is operatively mounted within the confines of each one link such that the link can then be deployed outboard of the associated windlass and fairlead assemblies for subsequent deformation causing the link parts to separate. Where manually removable fasteners are utilized, they are manually removed after (or before) each link has the explosive bolt assembly operatively set in to the link and before deployment and ignition. Where frangible fasteners are utilized, they are left in position so long as their fracture rating is such as to ensure that they will fracture when ignition of the explosive occurs.
Preferably, each one link includes a pair of similar, opposed U-shaped link parts having annular grooves formed therein near the free ends of the legs thereof and a pair of similar cooperating connector half parts each having opposed semicircular ridges movable laterally into a cooperating half of the associated annular grooves to retain the link parts together so long as the connector parts are retained together as by a pair of removable or frangible fasteners or alternatively by the bolt assembly capable of being explosively separated in response to a predetermined detonating signal. The application illustrates and describes a system wherein the detonation signal is hardwired to the explosive bolt assembly. With this arrangement, there is presented the disadvantages of wire handling and possible malfunctioning as a result thereof. While the aforesaid application indicates that acoustical detonation is contemplated, there is no disclosure as to how it would be accomplished.
One of the problems encountered in accomplishing this is to embody all of the acoustical receiver circuitry in the link. Existing receiver components available heretofore are not capable of being mounted within the confines of a link, and as a result would require a larger more expensive link that would also increase handling problems. Accordingly, it is an object of the present invention to provide low power consumption acoustical receiver circuitry capable of being provided within the confines of a link.
The present invention achieves this objective by providing an acoustical receiver and bolt assembly comprised of miniaturized acoustical receiving circuit, consuming very small amounts of power, for receiving a detonation signal, and a secnd class explosive which detonates when the receiving circuit receives the detonation signal. The use of a second class explosive is important because it provides an additional measure of safely in that it will be be detonated due to mishandling, i.e. dropping of the explosive assembly.
Additionally, the receiver portion of the assembly is designed to operate in conjunction with an acoustical transmitter which offers several more innovative safety features. First, the transmitter and receiver are designed for transmission and reception, respectively, of a signal composed of two frequencies. Thus the likelihood of misoperation, due to the receiver receiving a spurious signal, is substantially reduced. Secondly, the transmitter is designed for operation in a test mode, whereby the integrity and operability of each assembly can be verified prior to deployment. Thirdly, the transmitter also includes an arming mode, prior to the firing mode, in which a "handshake" is conducted between the transmitter and each one link, containing the assembly, which is to be detonated, to thereby verify that only specific desired links will be detonated. And finally, in actually transmitting the firing signal two operator actions must be performed within a predetermined period of time, thereby reducing the possibility of accidental detonation of any links.
Therefore, the present invention provides a separable link which is safely handled and deployed, in that after the assembly, containing the miniaturized, low power consumption receiving circuit and the secondary explosive, has been tested, it is mounted within the separable link which is then passed through the anchor windlass and fairlead assemblies, in order that it be deployed out at sea.
Another object of the present invention is the provision of an improved method of disengaging, under emergency conditions, a mooring system for a semi-submersible drilling rig or the like of the type described using the acoustical receiver and bolt assembly, containing the miniaturized receiver circuitry and secondary explosive as described above, which method comprises the steps of normally maintaining a separable link without the assembly in fastened relation between two links of each chain, in a position disposed in an associated chain locker when the associated anchor is deployed in its mooring position and then when the emergency conditions arise carrying out the following four steps: (1) mounting the assembly within each separable link (2) testing the assembly (3) operating the associated anchor windlass assembly to move the associated chain outwardly to deploy the separable link with the assembly mounted therein, and (4) detonating the assembly to separate the parts of the separable link, by transmitting the acoustical firing signal.
It will be appreciated by those skilled in the art that the present invention is not limited to the above-described application, but is capable of use anywhere where an acoustically linked transmitter and receiver are needed and a space problem exists with regard to the mounting or positioning of the acoustical receiver.
These and other objects of the present invention will become more apparent during the course of the following detailed description and appended claims: